This invention relates to a process for converting a paraffinic feed, e.g., a naphtha feedstock, to olefins and/or aromatics employing a particular low acidity zeolite catalyst.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (U.S. Pat. No. Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
Catalytic reforming of naphtha feedstocks has long been known in the petroleum industry. Most naphtha feeds contain large amounts of naphthenes and paraffins and consequently have low octane numbers. By means of various hydrocarbon conversion reactions, catalytic reforming has improved the octane number of naphtha feedstocks. Some of the more important conversion reactions that take place during catalytic reforming are dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to naphthenes and aromatics and isomerization of normal paraffins to isoparaffins. A less desirable reaction which also occurs during reforming is the hydrocracking of paraffins, naphthenes and dealkylation of alkylaromatics to gaseous hydrocarbons such as methane and ethane.
The above reforming reactions have previously been catalyzed by catalysts comprising porous supports, such as alumina, which possess dehydrogenation promoting metal components impregnated or admixed therewith. Platinum on alumina and, more recently, multimetallics, including bimetallics, such as platinum and rhenium on alumina, are examples of such catalysts. Representative multimetallic reforming catalysts are described in U.S. Pat. Nos. 2,848,377; 3,415,737; and 3,953,368, among others.
Conventional reforming catalysts, e.g., platinum and rhenium, are disclosed in U.S. Pat. No. 4,141,859 in admixture with a zeolite such as ZSM-5.
A reforming process is disclosed in U.S. Pat. No. 4,325,808 which utilizes as catalyst, a mixture of a noble-metal component, e.g., platinum, on a refractory inorganic oxide such as alumina and a non-noble metal-containing component, e.g., rhenium, on a large pore zeolite such as mordenite which has been disposed within a refractory inorganic oxide such as alumina.
Certain zeolites have also received considerable attention in recent years for their ability to catalyze the conversion of paraffins to aromatics.
According to U.S. Pat. No. 3,755,486, C.sub.6-10 hydrocarbons undergo dehydrocyclization to benzene and alkylbenzenes in the presence of a Li, Na or K zeolite X or Y or faujasite impregnated with 0.3 to 1.4 percent Pt.
U.S. Pat. No. 3,756,942 describes a process for the conversion of a feed containing liquid paraffins, olefins or naphthenes and mixtures thereof to aromatic compounds employing a porous synthetic synthetic crystalline silicate such as ZSM-5 as catalyst.
U.S. Pat. No. 3,760,024 discloses the aromatization of a feed containing C.sub.2-4 paraffins and/or olefins in the absence of added hydrogen employing ZSM-5 as catalyst.
According to U.S. Pat. No. 3,855,115, aromatization of hydrocarbons is accomplished employing rhenium-exchanged ZSM-5.
Aliphatic naphthas are upgraded to products of increased aromatics content by the process disclosed in U.S. Pat. No. 3,890,218. The process employs a zeolite catalyst such as ZSM-5 into which one or more metals which increase the aromatization activity of the zeolite, e.g., zinc or cadmium, have been incorporated.
In the process disclosed in U.S. Pat. No. 4,347,394, light straight-run naphthas and similar mixtures are converted to highly aromatic mixtures, principally benzene, employing a Group VIII metal-containing intermediate pore size zeolite, e.g., ZSM-5, which has been rendered substantially free of acidity by treatment with an alkali metal compound, e.g., NaOH.
Gaseous feedstocks containing ethane are converted to a mixture of benzene, toluene and xylene ("BTX") in the process of U.S. Pat. No. 4,350,835 utilizing a gallium-containing zeolite such as ZSM-5. A similar catalyst further containing thorium is disclosed in U.S. Pat. No. 4,629,818.
U.S. Pat. No. 4,435,283 describes a method for dehydrocyclizing alkanes employing as catalyst, a Group VIII metal-containing large pore zeolite which further contains an alkaline earth metal, e.g., zeolite X, Y or L containing Pt and barium.
In accordance with the process disclosed in U.S. Pat. No. 4,720,602, C.sub.2 to C.sub.12 aliphatic hydrocarbons are converted to aromatics over a zeolite catalyst, e.g., ZSM-5, which has been activated with zinc.